UNION & UNION ALL

UNION stacks the results of two SELECT queries. Columns must match in count and compatible types. UNION removes duplicate rows.

UNION — combine result sets (unique rows)

UNION stacks the results of two SELECT queries. Columns must match in count and compatible types. UNION removes duplicate rows.

Real-life example: UNION is merging two attendance sheets from morning and afternoon sessions into one master list without duplicate names.

Combine Delhi and Mumbai student names

SELECT full_name, city FROM students WHERE city = 'Delhi'
UNION
SELECT full_name, city FROM students WHERE city = 'Mumbai';

UNION ALL — keep duplicates

UNION ALL is faster when you know duplicates are fine or impossible. It keeps every row from both queries.

Real-life example: UNION ALL is photocopying both lists and stapling them — if a name appears twice, it stays twice.

All enrollments tagged by source query

SELECT student_id, 'Math track' AS track FROM enrollments WHERE course_id = 1
UNION ALL
SELECT student_id, 'Science track' AS track FROM enrollments WHERE course_id = 2;